Rigid tipped riblets

ABSTRACT

A multilayer construction for aerodynamic riblets includes a first layer composed of a material with protuberances, the first layer material exhibiting a first characteristic having long-term durability and a second layer composed of a material, exhibiting a second characteristic with capability for adherence to a surface.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/051,271 which is a continuation of U.S. patent application Ser. No.12/361,840 now U.S. Pat. No. 8,684,310 issued on Apr. 1, 2014, bothentitled Rigid Tipped Riblets and both having a common assignee with thepresent application, the disclosures of which are incorporated herein byreference.

BACKGROUND INFORMATION

1. Field

Embodiments of the disclosure relate generally to the field of surfacegeometries for aerodynamic improvements to aircraft or surfaces having aflow interface and more particularly to embodiments and fabricationmethods for rigid riblets having improved damage resistance.

2. Background

Increasing fuel efficiency in modern aircraft is being accomplishedthrough improvement in aerodynamic performance and reduction ofstructural weight. Recent advances in the use of microstructures such asriblets on aerodynamic surfaces have shown significant promise inreducing drag to assist in reducing fuel usage. Riblets have variousforms but advantageous embodiments may be ridge-like structures thatminimize drag on the surface of an aircraft. Riblets may be used inareas of a surface of an aircraft where turbulent regions may bepresent. Riblets may limit circulation causing a breakup of large scalevortices in these turbulent regions near the surface in the boundarylayer to reduce drag.

In certain tested applications, riblets have been pyramidal or invertedV shaped ridges spaced on the aerodynamic surface to extend along thesurface in the direction of fluid flow. Riblet structures have typicallyemployed polymeric materials, typically thermoplastics. However inservice use such as on an aircraft aerodynamic surface, polymers arerelatively soft and thus reducing the durability of the surface.Existing solutions with polymeric tips may readily deform hundreds ofpercent with fingernail pressure and may be unrecoverable. Suchstructures may be undesirable in normal service use on an aircraft orother vehicle. Additionally aircraft surfaces are typically required towithstand interactions with various chemicals including Skydrol®, ahydraulic fluid produced by Solutia, Inc. In certain applicationselastomers that resist or recover from severe deformation created at thetip may be employed to form the riblets. However, many elastomers andother polymers may not be compatible with Skydrol® or other aircraftfluids or solvents.

The practicality of riblets for commercial aircraft use would thereforebe significantly enhanced with a riblet structure providing increaseddurability and aircraft fluids compatibility.

SUMMARY

Exemplary embodiments provide a multilayer construction having a firstlayer composed of a material with riblets, the first layer materialexhibiting a first characteristic of having long term durability and asecond layer composed of a material exhibiting a second characteristicwith capability for adherence to a surface. The multilayer constructionis employed in exemplary embodiments wherein the riblets are implementedon a vehicle, the riblets having long-term durability due to therigidity of the first layer.

In various embodiments, the multilayer construction for an array ofaerodynamic riblets is created by a plurality of rigid tips with a layersupporting the rigid tips in predetermined spaced relation and adheringthe rigid tips to a vehicle surface. In exemplary embodiments, the rigidtips are formed from material selected from the set of nickel, chromium,metal alloy, glass, ceramic, silicon carbide and silicon nitride.Additionally, the supporting layer may be continuously cast with thetips as a surface layer. Alternatively, a polymer support layer isdeposited on the surface layer opposite the tips. An adhesive layerdeposited on the polymer support layer forms a multilayer appliqué, andprovides the capability for adhering the appliqué to the vehiclesurface.

In another exemplary embodiment, the supporting layer is an elastomericlayer engaging the tips and a metal foil and a polymer layer areprovided intermediate the elastomeric layer and the adhesive layer. Themetal foil polymer layer and adhesive layer may be provided as apreformed appliqué. For exemplary embodiments using the elastomericlayer, the tips each incorporate a base and each base may be embedded inthe elastomeric layer.

For greater flexibility in certain applications, each tip islongitudinally segmented.

An aircraft structure may be created by an array of aerodynamic ribletshaving a plurality of rigid tips formed from material selected from theset of nickel, chromium, metal alloy, glass, ceramic, silicon carbideand silicon nitride and segmented longitudinally at predeterminedlocations. An elastomeric layer engages bases extending from the rigidtips and a polymer support layer is deposited on the elastomeric layeropposite the tips. An adhesive layer deposited on the polymer supportlayer to forms a multilayer appliqué. The adhesive layer adheres to asurface of the aircraft.

The embodiments disclosed are fabricated in an exemplary method byforming a master tool having protuberances corresponding to a desiredriblet array and forming a complementary tool from the master tool. Aplurality of rigid tips is then cast in the master tool usingelectroforming, casting or other desirable deposition technique. Thecast rigid tips are then removed from the complementary tool and adheredto an aerodynamic surface.

In exemplary aspects of the method, resist is applied to thecomplementary tool for a segregating the rigid tips and removedsubsequent to casting the rigid tips. An elastomeric layer is then castengaging the rigid tips and a multilayer appliqué is applied to theelastomeric layer to form a riblet array appliqué.

In exemplary embodiments of the method, the multilayer appliquécomprises a metal foil, a polymer support layer and an adhesive layer.An adhesive liner covering the adhesive layer and masking covering theriblets may be employed for handling. The riblet array may then beadhered to the aerodynamic service by removing the adhesive liner andapplying the riblet array appliqué to the aerodynamic surface andremoving the masking.

In an alternative method, casting the plurality of rigid tips includescasting of the plurality of tips and an intermediate surface layer as acladding. An elastomeric layer is then cast to the cladding.

A method for fabricating an array of aerodynamic riblets for an aircraftsurface may be accomplished by diamond machining a form and curing anacrylate film on the form. The acrylate film is then stripped from theform and applied to a roller to form a master tool having protuberancescorresponding to a desired riblet array. A silicon complementary webtool is created by impression on the master tool. A metal coating isthen sputtered on the complimentary web tool and a plurality of rigidtips is then electroformed in the complimentary web tool. A multilayerappliqué having a metal foil, a polymer support layer and an adhesivelayer to the elastomeric layer is applied to form a riblet arrayappliqué. The rigid tips are then adhered to an aerodynamic surfaceusing the adhesive layer of the appliqué and the silicone complementaryweb tool is then stripped from the rigid tips.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of embodiments disclosed herein will bebetter understood by reference to the following detailed descriptionwhen considered in connection with the accompanying drawings wherein:

FIG. 1 is an isometric view of a portion of an aerodynamic surface suchas a wing or fuselage skin showing exemplary riblets extending in theflow direction;

FIG. 2A is a lateral section view perpendicular to the flow direction ofa first embodiment for rigid tipped riblets;

FIG. 2B is a lateral section view of a modification of the embodiment ofFIG. 2A with an additional support layer;

FIG. 2C is a lateral section view of a modification of the embodiment ofFIG. 2A with rigid cladding over an elastomer core;

FIG. 2D is a lateral section view of a modification of the embodiment ofFIG. 2A without an adhesive layer for direct thermoplastic boding;

FIG. 3 is a lateral section view of a second embodiment for rigid tippedriblets with lateral structural separation of the riblets;

FIG. 4 is a lateral section view of a third embodiment for rigid tippedriblets with reduced cross-section and with lateral separation;

FIG. 5A is a top view of a portion of an aerodynamic surface employingriblets of the first embodiment as shown in FIG. 2B;

FIG. 5B is a section view comparable to FIG. 2B for reference with thefeatures of FIG. 5A;

FIG. 6A is a top view of a portion of an aerodynamic surface employingriblets of the second embodiment shown in FIG. 2B with additionallongitudinal separation of riblet sections;

FIG. 6B is a section view comparable to FIG. 4 for reference with thefeatures of FIG. 6A;

FIG. 7A is a flow diagram of processing steps for a first exemplarymethod of fabrication of rigid tipped riblets of the first embodiment;

FIG. 7B is a flow diagram of processing steps for a second exemplarymethod of fabrication of rigid tipped riblets of the first embodiment;

FIG. 7C is a flow diagram of roll-to-roll processing for the methodshown in FIG. 7B

FIG. 8 is a flow diagram of processing steps for an exemplary method offabrication of rigid tipped riblets of the second embodiment;

FIG. 9 is a flow diagram of processing steps for an exemplary method offabrication of rigid tipped riblets of a third embodiment;

FIG. 10 is a flow diagram describing use of the rigid tipped ribletsembodiments disclosed herein in the context of an aircraft manufacturingand service method; and

FIG. 11 is a block diagram representing an aircraft employing the rigidtipped riblets with embodiments as disclosed herein.

DETAILED DESCRIPTION

An exemplary embodiment of rigid tipped riblets having a structure aswill be described in greater detail subsequently is shown as a portionof an aerodynamic surface for an aircraft is shown in FIG. 1. “Rigid” asused herein generally refers to a high modulus of elasticity and/or ahigh load to failure. Many of these materials may have a small strainelastic region. Exemplary embodiments herein employ rigid materialswhich may have moduli of elasticity up to and larger than about 25×10⁶lbs/in² with deformation response essentially all elastic. The aircraft110 employs a structure with a surface 111, shown enlarged, havingmultiple substantially parallel riblets 112 arranged parallel to theflow direction as represented by arrow 114. For the exemplary embodimentshown, the dimension 116 perpendicular to the surface 111 (as shown inFIGS. 2A and 2B for example) is approximately 0.002 inch while thetip-to-tip spacing 118 between the riblets is approximately 0.003 inch.Spacing may vary depending on the fluid dynamic properties of the air,water or other fluid for which the application of riblets is employed.The aerodynamic surface is typically curved and may be, withoutlimitation, a portion of a wing, an engine nacelle, a control surface, afuselage or other suitable surface. Therefore flexibility andconformability of the riblets and any structure supporting and affixingthe riblets to the surface may be required. While described herein withrespect to an aircraft aerodynamic surface the embodiments disclosedherein are equally applicable for drag reduction on surfaces of otheraerospace vehicles such as, without limitation, missiles or rockets andother vehicles such as cars, trucks, buses and trains moving in agaseous fluid, commonly air, or on boats, submarines, hydrofoils, fluidflow conduits or other surfaces exposed to liquid fluid flow.

The embodiments disclosed herein recognize and provide the capabilityfor riblets that may resist various impacts and/or other forces that mayreduce riblet durability. Further, certain of the different advantageousembodiments provide a multi-layer structure that may have a supportlayer and a plurality of riblet tips located on or extending from thesupport layer. The tips which form the riblets may be fabricated fromstiff metals such as nickel (used for the embodiments described herein)or alternative rigid materials such as chromium, other metal alloys,glass, ceramics, Silicon Carbide or Silicon Nitride. The materials ofthe multilayer structure are flexible and may be formed as an appliquéseparately or in combination with the riblets for fastening, bonding,coupling or otherwise attaching to a surface to improve aerodynamics ofa vehicle such as an aircraft.

A first embodiment for rigid tipped riblets is shown in FIG. 2A as amultilayer construction. Individual tips 202 of the riblets protrudefrom a surface layer 204 to provide a first layer 201 of the multilayerconstruction. The protruding riblets and continuous surface layer areformed by casting or deposition, as will be described in greater detailsubsequently, of the rigid material desired for providing a firstcharacteristic of durability. In an exemplary embodiment, nickel isemployed. For the embodiment shown in FIG. 2A a second layer 203 createdby an adhesive layer 206 is deposited on a bottom 204 a of the surfacelayer 204. Exemplary adhesives for use in various embodiment mayinclude, without limitation, acrylic pressure sensitive adhesive,sylilated polyurethane pressure sensitive adhesive; thermoplasticadhesive; heat-reactive adhesive or epoxy adhesive. In alternativeembodiments, a supporting polymer layer 207 engages the surface layer204 intermediate the surface layer and adhesive layer as shown in FIG.2B as a portion of the second layer. The polymer layer 207 may be,without limitation, a polymer film or other suitable material. Incertain embodiments polyetheretherketone (PEEK) is employed as the film.Additionally, a foil or metallic layer 310 as will be described withrespect to the embodiment of FIG. 3 may be employed for lightning strikeprotection, particularly where the riblet tips 202 and surface layer 204are non-metallic. The polymer, adhesive and/or other elements in thesecond layer provide a second characteristic of resilience and theability to adhere to the surface.

FIG. 2C is an additional alternative embodiment wherein the nickel oralternative rigid material is employed as a contoured surface cladding208 forming the tips 202′ and surface layer 204′ as the first layer ofthe multilayer construction. As the second layer, a polymer layer 210 isemployed. The polymer layer 210 in certain embodiments as describedherein may be an elastomer and may be cast into the cladding 208 orconversely the cladding 208 cast over the polymer layer 210. The polymerlayer 210 provides both a support layer 206′ and light weight cores 212for the tips 202′ to maintain the predetermined spaced relation of thetips 202′. Exemplary elastomers used in exemplary embodiments may bepolyurethane elastomers, polysulfide elastomers, epoxy-based elastomers,silicones, fluoroelastomers, fluorosilicone elastomers, EPDM elastomers,or other polymers with lower strain to yield, for example thermoplasticpolyurethanes, PEEK, PEKK or polyamide. This alternative embodiment mayallow weight reduction and flexibility of the structure may be furtherenhanced. The polymer layer 210 may then be adhered to a surface usingan adhesive layer 206 or directly as described with respect to FIG. 2D.

In the form shown in FIG. 2A, 2B or 2C, the embodiment may fabricated asa multilayer appliqué 209, as shown in FIG. 2B, including the tips 202,surface layer 204, polymer layer 207 and adhesive layer 206 which canthen be adhered to the aerodynamic surface 111 using the adhesive layer206.

In alternative embodiments, the surface layer 204 may be directlyadhered to or deposited on an aircraft surface 111. FIG. 2D demonstratesan embodiment similar to that described with respect to FIG. 2C however,no adhesive layer is employed. Elastomeric layer 210′ is a thermoplasticcast into the nickel cladding 208 which allows direct bonding to theaircraft surface 111 with application of heat.

Another embodiment for rigid tipped riblets is shown in FIG. 3. Withcomplex or multiple curved surfaces, it may be desirable in the firstlayer 301 for the individual riblet tips 302 to be separated from eachother perpendicular to the flow direction for greater lateralflexibility. For the embodiment shown individual tips 302 protrude froman elastomeric layer 304. Tips 302 have an internal angle 303 ofapproximately 300 for the exemplary embodiment. A base 306 expands fromeach tip. In certain embodiments the elastomeric layer 304 surrounds thebase 306 to provide greater structural continuity. In alternativeembodiments a bottom face 308 of the base adheres directly to theexposed surface 304 a of the elastomeric layer 304.

The second layer 303 is created by a multilayer structure incorporatinga screen and/or foil metallic layer 310 such as aluminum, a polymerlayer 312 such as PEEK and an adhesive layer 314 supports theelastomeric layer 304. The polymer layer 312 and adhesive layer 314 maybe supplied as a portion of the preformed appliqué as described withrespect to FIG. 9 below or directly deposited on the elastomeric layer304. The metallic layer 310 provides a conducting material for lightningstrike protection in an exemplary aircraft usage of the embodiment. Themetallic layer, polymer and adhesive multilayer structure may becomparable to a current lightning strike appliqué (LSA) employed forcomposite aircraft structural surfaces.

The elastomer layer 304 supporting the riblet tips 302 may provideelastic sideways deformation and recovery for the tips 302 when lateralforces are applied thereby further enhancing the durability of the rigidriblet tips. Additionally, the elastomeric layer 304 flexibility mayallow greater ability to conform to complex contour shapes.

FIG. 4 demonstrates a third embodiment for the rigid tipped riblets 112in FIG. 1 which takes advantage of the structural capability provided bythe material from which the riblets 112 are formed to allow a sharperprofile of tips 402. For the embodiment shown in each of the tips 402extends from a base 406 supported in an elastomer layer 404. As with theembodiment described with respect to FIG. 3 the base 406 of each tip 402is surrounded by the elastomer to structurally retain the base 406within the elastomer layer 404. In alternative embodiments, the extendedbottom surface 408 of the base 406 may be adhered to the surface 404 aof the elastomer layer 404. The embodiment of FIG. 4 also employs riblettips 402 separated perpendicular to the flow direction 114 as in theembodiment of FIG. 3. However, in alternative embodiments a continuoussurface layer 204 from which the tips 202 extend as disclosed for theembodiment described with respect to FIG. 2A may be employed.

As also disclosed in FIG. 4 the embodiment employs a supporting polymerlayer 410 on which the elastomer layer 404 is adhered or deposited. Anadhesive layer 412 extends from the polymer layer opposite the elastomerlayer 410 forming a multilayer appliqué 414.

FIG. 5 shows a top view of the embodiment as disclosed in FIG. 2B. Theriblets formed by the tips 202 extend longitudinally along surface layer204 in the flow direction 114. The thin surface layer 204 provides forflexibility in adhering to curvature having tangents substantiallyperpendicular to the riblets. However as previously described, thesurface 111 on which the riblets 112 may be employed may have multiplecomplex curvatures requiring greater flexibility. The embodimentspreviously described may therefore be adapted as shown in FIG. 6Awherein the individual tips 402 as described with respect to FIG. 4 arelaterally separated by spacing 118 substantially perpendicular to theflow direction 114 with bases 406 attached to or captured within anelastomer layer 404. This provides even greater flexibility for adheringto surfaces with curvatures having tangents (generally shown asrepresented by arrow 604) substantially perpendicular to the riblets112. The scale of the drawings herein based on the small ribletdimensions makes the surfaces appear flat even though they may be curvedin larger scale. An aluminum foil layer 407 has been added to theembodiment of FIG. 6B for demonstration of an embodiment for lightningstrike protection with tips 402 which may be non-metallic. Additionallythe individual riblets incorporate longitudinal separation in the flowdirection using gaps 602 to segment the riblet to provide greaterflexibility for adhering to surfaces having curvatures with tangentssubstantially parallel to the riblets 112 in the flow direction 114. Forthe embodiment shown gaps 602 may be evenly spaced in the riblets 112 atsubstantially equal longitudinal distances 606. In alternativeembodiments spacing on individual riblets 112 and between riblets 112may be uneven and chosen in a predetermined manner to accommodatesurface curvature as required.

FIG. 7A is a flow diagram showing a first exemplary manufacturingprocess for a riblet structure as defined in the embodiment describedwith respect to FIG. 2A. In step 701 a master tool 712 is created using,as an example without limitation, diamond machining of a copper form orother suitable material on which an acrylate film is cured then strippedto define spaced protuberances 714 corresponding to the desired ribletdimensions. The tool 712 as shown in FIG. 7A may be a section of a flattool, or a roller employed for roll-to-roll web processing. Exemplarydetails of a web processing format are shown in FIG. 7C. For theembodiment shown in FIG. 7A nickel is employed for the rigid tips 202. Acomplimentary tool 716 is created in step 702 by impression, casting orelectroforming on the master tool 712 which provides grooves 718corresponding to the riblet shape. Spacing between the grooves 718provides a substantially flat intermediate surface 720 corresponding tothe dimension 118 desired between the tips 202. In step 703, rigid tips202 and surface layer 204 may be deposited by electro-forming onto thecomplimentary tool 716. In certain embodiments, a release compound isapplied to the surfaces on the complimentary tool to assist in removalof the cast riblets and surface layer from the tool. Adhesive layer 206is then applied, in step 704, to the surface layer 204 opposite therigid tips 202. The adhesive layer 206 may be combined with a polymerlayer, such as support layer 207 as shown in FIG. 2B and supplied as apreformed appliqué which is then joined with the electroformed surfacelayer 204. A removable adhesive liner 722 for handling of the completedappliqué is added as also shown in step 704. The appliqué, created bysurface layer 204 and adhesive layer 206, is removed from thecomplimentary tool 716 and a masking layer 724 is applied for handlingas shown in step 705. For exemplary embodiments, the masking employedmay be, without limitation, static masking films, masking films with lowtack pressure sensitive adhesive, or castable films of silicone.Application to the aircraft surface 726 is accomplished by removal ofthe adhesive liner 722 followed by attachment of the adhesive layer 206of the appliqué to aircraft surface 726. Removal of the masking layer724 completes the riblet appliqué processing.

The complimentary tool 716 may be a “web tool” which may be silicone orpolymeric film. Roll-to-roll processing for the steps describedsubsequently may then be employed as shown in FIG. 7C and the web tool716 may be left in place as the masking that is removed afterinstallation of the array of riblets 112 on the aircraft surface 726. Asshown in FIG. 7B for a method employing the web tool approach, a mastertool 712 is created in step 731 define spaced protuberances 714corresponding to the desired riblet dimensions. The tool 712 as shown inFIG. 7B may be a section of a flat tool, or a roller employed forroll-to-roll web processing. A complimentary web tool 746 is created instep 732 by roll processing silicone on the master tool 712 whichprovides grooves 718 corresponding to the riblet shape. Spacing betweenthe grooves provides a flat intermediate surface 720 corresponding tothe dimension 118 desired between the rigid tips 202. A conductivelayer, shown as the dashed line designated as element 747, is thensputtered onto the silicon web tool, in step 733, providing a conductivesurface on the web tool. In step 734, rigid tips 202 and surface layer204 are deposited by electro-forming onto the web tool. Adhesive layer206 is then applied in step 735 to the surface layer 204 opposite therigid tips 202. The adhesive layer 206 may be combined with a polymerlayer 207, as shown for the embodiment in FIG. 2B, and supplied as apreformed appliqué 723 which is then joined with the electroformedsurface layer 204. A removable adhesive liner 722 for handling of thecompleted appliqué 723 is added as also shown in step 735. Applicationto the aircraft surface 724 is accomplished by removal of the adhesiveliner 722 shown in step 736 followed by attachment of the adhesive layer206 of the appliqué to aircraft surface 724 in step 737. Stripping ofthe silicone web tool 746 exposes the rigid tips 202 of the riblets andcompletes the riblet appliqué processing.

As shown if FIG. 7C, a roll-to-roll web processing approach may beemployed for the methods described. Master tool 712 is created using, asan example, diamond machining of a copper form 742 on which an acrylatefilm 744 is cured then stripped and applied to a roller 745 to providethe master tool 712 shown in the drawing. Complimentary web tool 746 isthen created by impression on master tool 712. Conductive layer 747 issputtered onto the web tool 746 using sputtering gun 750 andelectroforming of the tips 202 surface layer 204, as shown for examplein FIG. 7B, onto the web tool 746 is accomplished with deposition tool752. The adhesive layer 206 is then deposited on the surface layer 204with deposition tool 754 and the removable adhesive liner 722 attachedby application from roll 756. The multilayer appliqué 725 is thenavailable for attachment to the aircraft surface 724 as shown, forexample, in step 737 of FIG. 7B.

FIG. 8 is a flow diagram showing a manufacturing process for a ribletstructure as defined in the embodiment described with respect to FIG. 3.In step 801 a web tool 812 is created as previously described withrespect to FIG. 7C to define spaced protuberances 814 corresponding tothe desired riblet dimensions. The tool 812, as shown in FIG. 8, may bea section of a flat tool or a roll tool employed for web processing. Forthe embodiment shown in FIG. 8, nickel is employed for the rigid tips302. A complimentary tool 816 is created in step 802 by impression onthe web tool 816 which provides grooves 818 corresponding to the ribletshape. Spacing between the grooves provides a substantially flatintermediate surface 820 corresponding to the dimension 118 desiredbetween the riblet tips 302. In certain embodiments, the complimentarytool 816 may be nickel or a silicon web tool as described with respectto FIG. 7C. In step 803 resist 822 is applied to the flat surfaces 820on the nickel tool and rigid tips 302 are deposited by electro-formingonto the tool in step 804. The resist 822 is then removed in step 806providing the spaced riblets in the tool. For the embodiment shown thebases 306 are placed into relief extending from the tool 816 by theremoval of the resist as shown in step 806. The elastomer layer 304 isthen cast over the bases 306 in step 807. In alternative embodimentselectroforming of the rigid tips 302 provides a base substantially flushwith the flat surface for direct adherence to the elastomer surface 305as previously described with respect to FIG. 3. For the exemplaryprocess shown with respect to FIG. 8 a preformed appliqué 824 comprisingthe multilayer structure of aluminum foil as a metallic layer 310,polymer layer 312 and adhesive layer 314 is adhered to the castelastomer 304 in step 808. A removable adhesive liner 826 forpreservation of the adhesive during further processing is shown as aportion of the preformed appliqué. The multilayer structure is thenremoved from the complimentary tool 816 creating a multilayer ribletarray appliqué 829 and exposing the rigid tips 302. Masking 828 isapplied over the tips 302 and elastomer 304 to assist in handling duringadditional processing and as also shown in step 808. The masking 828 inexemplary embodiments may be, without limitation, a solution castreleasable polymer such as silicon or an adhesive film such as Mylar®with a low tack acrylic adhesive applied during roll processing.Alternatively, the complimentary web tool 816 when fabricated from awater/fluid soluble polymer may be employed as masking layer 828 toallow removal of the masking by dissolving with water or other fluidafter installation.

The completed multilayer riblet array appliqué 829 may then be appliedto an airplane surface 830 by removing the adhesive liner 826 andadhering the adhesive layer 314 to the surface 830 as shown in step 809.The masking 828 is then removed from the tips 302 and elastomer 304.

The rigid materials employed for the tips as described in theembodiments and fabrication processes herein allows very fine tipstructure having a dimension 307 of around 15 to 25 microns at the basewith a dimension 309 at the extreme end of the tips typically on theorder of 100 nanometers (0.1 micron) as shown in FIG. 3. Smaller tipsmay be obtained with tooling and release process refinement. Eventhought the tips are very sharp, the very fine spacing of the tipsavoids cuts in normal handling by installation personnel.

FIG. 9 is a flow diagram showing a manufacturing process for a ribletstructure as defined in the embodiment described with respect to FIG.2A. In step 901 a master tool 912 is created. The tool 912, as shown inFIG. 9, may be a section of a flat tool or a roller employed forroll-to-roll web processing. For the embodiment shown in FIG. 9 nickelis employed for the cladding 208 which forms the rigid tips 202′ andsurface layer 204′. A complimentary tool 916 is created in step 902 byimpression on the master tool 912 which provides grooves 918corresponding to the riblet shape. Spacing between the grooves providesa substantially flat intermediate surface 920 corresponding to thedimension 118 desired between the riblets tips 202′. In step 903 nickelcladding 208 is deposited by electroforming into the complimentary tool916 to form rigid tips 202′ and surface layer 204′ in step 903. Inalternative embodiments, the cladding may be cast or roll formed intothe complimentary tool. In certain embodiments, a release compound isapplied to the surfaces on the complimentary tool 916 to assist inremoval of the tips 202′ and surface layer 204′ from the tool 916.Polymer layer 210 is then cast into the cladding 208 to provide both asupport layer and light weight cores 212 for the tips in step 904. Aspreviously described the polymer layer 210 may be an elastomer incertain embodiments. Adhesive layer 206 is then applied in step 905 tothe polymer layer 210 opposite the rigid tips 202′ to create an appliqué922. A removable adhesive liner 924 for handling of the completedappliqué 922 is added, the appliqué 922 with adhesive liner 924 isremoved from the nickel tool 916 and masking 926 is applied over thetips 202′ and surface layer 204′ for handling as also shown in step 905.Application to the aircraft surface 928 is accomplished as shown in step906 by removal of the adhesive liner 924 followed by attachment of theadhesive layer 206 of the appliqué 922 to aircraft surface 928. Removalof the masking 926 completes the process.

Referring more particularly to FIGS. 10 and 11, embodiments of the rigidriblets disclosed herein and the methods for their fabrication may bedescribed in the context of an aircraft manufacturing and service method1000 as shown in FIG. 10 and an aircraft 1102 as shown in FIG. 11.During pre-production, exemplary method 1000 may include specificationand design 1004 of the aircraft, which may include the riblets, andmaterial procurement 1006. During production, component and subassemblymanufacturing 1008 and system integration 1010 of the aircraft takesplace. The riblet appliqués and their manufacturing processes asdescribed herein may be accomplished as a portion of the production,component and subassembly manufacturing step 1008 and/or as a portion ofthe system integration 1010. Thereafter, the aircraft may go throughcertification and delivery 1012 in order to be placed in service 1014.While in service by a customer, the aircraft 1002 is scheduled forroutine maintenance and service 1016 (which may also includemodification, reconfiguration, refurbishment, and so on). The ribletappliqués as described herein may also be fabricated and applied as aportion of routine maintenance and service.

Each of the processes of method 1000 may be performed or carried out bya system integrator, a third party, and/or an operator (e.g., acustomer). For the purposes of this description, a system integrator mayinclude without limitation any number of aircraft manufacturers andmajor-system subcontractors; a third party may include withoutlimitation any number of venders, subcontractors, and suppliers; and anoperator may be an airline, leasing company, military entity, serviceorganization, and so on.

As shown in FIG. 11, the aircraft 1102 produced by exemplary method 1000may include an airframe 1118 having a surface 111, as described withrespect to FIG. 1, and a plurality of systems 1120 and an interior 1122.Examples of high-level systems 1120 include one or more of a propulsionsystems 1124, an electrical and avionics system 1126, a hydraulic system1128, and an environmental system 1130. Any number of other systems maybe included. The rigid tipped riblets supported by the embodimentsdisclosed herein may be a portion of the airframe, notably the finishingof skin and exterior surfaces. Although an aerospace example is shown,the principles disclosed by the embodiments herein may be applied toother industries, such as the automotive industry and the marine/shipindustry.

Apparatus and methods embodied herein may be employed during any one ormore of the stages of the production and service method 1000. Forexample, components or subassemblies corresponding to production process1008 may be fabricated or manufactured in a manner similar to componentsor subassemblies produced while the aircraft 1102 is in service. Also,one or more apparatus embodiments, method embodiments, or a combinationthereof may be utilized during the production stages 1008 and 1010, forexample, by substantially expediting assembly of or reducing the cost ofan aircraft 1102. Similarly, one or more of apparatus embodiments,method embodiments, or a combination thereof may be utilized while theaircraft 1102 is in service, for example and without limitation, tomaintenance and service 1016.

Having now described various embodiments in detail as required by thepatent statutes, those skilled in the art will recognize modificationsand substitutions to the specific embodiments disclosed herein. Suchmodifications are within the scope and intent of the present disclosureas defined in the following claims.

What is claimed is:
 1. A method for fabricating an array of ribletscomprising: forming a master tool having protuberances corresponding toa desired riblet spacing: forming a complementary tool from the mastertool; depositing rigid tips in the complimentary tool; removing therigid tips from the complementary tool; integrally depositing a surfacelayer intermediate the rigid tips; and depositing an adhesive layer onthe surface layer.
 2. The method of claim 1 further comprising:depositing a polymer layer on the surface layer opposite the rigid tips.3. The method of claim 1 further comprising: applying resist to thecomplementary tool for a segregating the rigid tips; and removing theresist subsequent to casting the rigid tips.
 4. The method of claim 3further comprising: casting an elastomeric layer engaging the rigidtips.
 5. The method of claim 4 further comprising: applying a preformedappliqué to the elastomeric layer to form a riblet array appliqué. 6.The method of claim 5 wherein the preformed appliqué comprises ametallic layer, a polymer layer and an adhesive layer.
 7. The method ofclaim 6 further comprising: applying adhesive liner to the adhesivelayer; removing the riblet array appliqué from the complementary tool;applying masking to the rigid tips.
 8. The method of claim 7 furthercomprising: removing the adhesive liner and applying the riblet arrayappliqué to the surface; and removing the masking.
 9. The method ofclaim 1 wherein the step of depositing the rigid tips includes castingof a plurality of tips and an intermediate surface layer as a claddingand further comprising casting an elastomeric layer to the cladding. 10.The method of claim 1 wherein the step of depositing the plurality ofrigid tips comprises electro-forming the rigid tips on the complementarytool.
 11. A method for fabricating an array of aerodynamic riblets foran aircraft surface comprising: diamond machining a form; curing anacrylate film on the form; stripping the acrylate film from the form andapplying the film to a roller to form a master tool having protuberancescorresponding to desired riblet array dimensions; forming acomplementary tool from the master tool; electroforming a plurality ofrigid tips in the complimentary tool; casting an elastomeric layer overbases of the rigid tips; applying a preformed appliqué having a metalliclayer, a polymer layer and an adhesive layer to the elastomeric layer toform a multilayer riblet array appliqué; removing the multilayer ribletarray appliqué from the complimentary tool; applying masking over therigid tips; adhering the rigid tips to a surface on an aircraft; and,removing the masking from the rigid tips.
 12. A method for improvingaerodynamic performance of an aircraft comprising: depositing a rigidmaterial on a tool to form rigid tips; integrally depositing a surfacelayer on the tool intermediate the rigid tips; removing the rigid tipsand surface layer from the complementary tool; depositing an adhesivelayer on the surface layer; and, bonding the adhesive layer to a surfaceof an aircraft.
 13. The method for improving aerodynamic performance ofan aircraft as defined in claim 12 wherein the step of integrallydepositing a surface layer comprises: casting an elastomeric layerengaging the rigid tips;
 14. The method for improving aerodynamicperformance of an aircraft as defined in claim 13 wherein the step ofdepositing an adhesive layer includes: applying a preformed appliqué tothe elastomeric layer to form a riblet array appliqué; and the step ofbonding the adhesive layer comprises: bonding the riblet array appliqueto the surface of the aircraft.
 15. The method for improving aerodynamicperformance of an aircraft as defined in claim 14 wherein the preformedappliqué comprises a metallic layer, a polymer layer and an adhesivelayer.
 16. The method for improving aerodynamic performance of anaircraft as defined in claim 15 further comprising: applying adhesiveliner to the adhesive layer; removing the riblet array appliqué from thecomplementary tool; applying masking to the rigid tips.
 17. The methodfor improving aerodynamic performance of an aircraft as defined in claim16 further comprising: removing the adhesive liner and applying theriblet array appliqué to the surface; and removing the masking.
 18. Themethod for improving aerodynamic performance of an aircraft as definedin claim 12 wherein the rigid material comprises nickel or chromium. 19.The method for improving aerodynamic performance of an aircraft asdefined in claim 12 wherein the rigid material comprises glass,ceramics, Silicon Carbide or Silicon Nitride.
 20. The method forimproving aerodynamic performance of an aircraft as defined in claim 12further comprising: diamond machining a form; curing an acrylate film onthe form; stripping the acrylate film from the form and applying thefilm to a roller to form a master tool having protuberancescorresponding to desired riblet array dimensions; forming acomplementary tool from the master tool; and the step of depositing arigid material on a tool comprises depositing the rigid material on thecomplementary tool.